Abstracts 309

modalities over the next 5-10 years. It seems to be

important to work closely together during this evalua- tion period.

Low-Field Magnetic Resonance Imaging-Clinical Experiences

R. E. Sepponen

Helsinki University Central Hospital, NMR Labora- tory and Instrumentarium Corp., Helsinki, Finland

The effect of the field strength on the contrast of MR images is well recognized. The increase of the strength of the polarizing magnetic field increases the signal-

to-noise ratio and improves the resolution of the final image [I]. However, the effectivity of various relaxa- tion mechanisms changes with the resonance frequen-

cy. The relaxation processes due to the macromolecu- lar movements are effective at low resonance frequen- cies [ 21. The diagnostic effectivity of low-field imaging has been demonstrated [3,4, 51.

A prototype unit (ACUTSCANB, manufacturer Instrumentarium Corp., Helsinki, Finland) operating at the field strength 0.02 T (corresponding proton resonance frequency 0.8 MHz) was used. Inversion recovery and spin-echo sequences were used in order to generate T, and T, weighted images. The imaging

method was 2-DFT. The data acquisition was carried out as a spin echo. The selective excitation method was used to define the imaging plane. In order to reduce the imaging time the multislice technique was utilized.

The benefits of the low-field operation may be summarized as follows:

(1) Relaxation time T, of tissues at low field reflects macromolecular differences of tissues. The tissue contrast is demonstrated in the diagnosis of intracranial hematomas. At the operating field strength of the unit the relaxation time T, of freshly

extravasated blood is shorter than the T, of the surrounding brain tissue. This facilitates the differen- tial diagnosis of acute hemorrhage from other pathol-

ogies which in the most cases has long relaxation times T, and T, [5]. At 0.2 T and above the relaxation time of blood is equal or longer than that of the surrounding brain tissue [5, 61.

(2) The fringe fields are small and the missile effects are virtually eliminated. The 0.5 mT field extends 1.2 m from the ends of the solenoid and is usually well confined in the installation room. Thus

usually no special safety measures are needed against inadvertent magnetic field exposures (e.g. pacemaker patients).

(3) The unit may be constructed based on inexpen-

sive resistive magnet technology. The magnet is a solenoid with end-correction coils. The absolute inho-

mogeneities are small and thus long data collection

(4) The unit may be installed without extensive site preparation. The weight of the magnet is 850 kg and the total system weight is 1550 kg. The total power consumption is 7 kW and typical water consumption for cooling of the system is 4-6 l/min. Due to the small fringe fields magnetic shielding is not needed.

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Hart, H.R., Bottomley, P.A., Adelstein, W.A., et al., AJR 141:1195-1201, 1983. Borcard, B., Prog. Nucl. Med. 8~41-54, 1984. Hutchison, J.M.S., Smith, F.W., Partain, CL.. James, A.E., Rollo, F.D., Price, R.R., eds. “Nuclear Magnetic Resonance (NMR) Imaging”, pp. 23 1-249. Philadelphia, PA: W.P. Saunders, 1983. Sepponen, R.E., Sipponen, J.T., Sivula, A., J. Comput. Assist. Tomogr. 9~237-241, 1985. Sipponen, J.T., Sepponen, R.E., Tanttu, J.I., Sivula, A., J. Comput. Assist. Tomogr. (In press) 1985. DeLaPaz, R.L., New, P.F.J., Buonanno, F.S., et al., J. Comput. Assist. Tomogr. 8599-607, 1984.

time may be used in order to maximize the signal-

to-noise ratio.

Chemical Shift Imaging, Technical Requirements

Derek Shaw

I.G.E. Medical Systems, 260, Bath Road, Slough, England

The technical requirements for chemical shift imaging, which for the purposes of this talk will be defined as

procedures which produce localised spectroscopic information (i.e. M, x, y, 6) can be summarised as follows: a high, homogeneous magnetic field, a method of localisation, and a method of display.

The first of these requirements is self-evident; the field must be high enough to produce sufficient chemi- cal shift dispersion (>l T) and uniform enough to permit the lines of interest to be resolved (to.5 ppm). The third requirement depends on whether the data is

displayed as an image of a specific chemical shift, i.e. M, (x, y) or as a spectrum from a specific location, i.e. M,,(6). The next most challenging requirement is the method of localisation. The talk will concentrate on

this aspect of the topic. The first major subdivision of localisation methods

which can be made is between those methods which use linear field gradients (i.e. imaging techniques) to pro- vide the required localisation and those which do not.

Non-Linear Field Gradient Localisation Techniques Techniques in this group depend basically on the

properties of surface coils [ 11. The size and shape of the coil provide the primary source of iocalisation, the signal arising from an approximately hemispherical region with the same radius as the coil (for a circular coil). The problem axis with the surface coils is the one perpendicular to the plane. Depth resolution with surface coils can be achieved by simple surface nulling

310 Magnetic Resonance Imaging 0 Volume 3, Number 3, 1985

(i.e. 180” pulse at surface), topical magnetic resonance (TMR) [2] depth pulses [3], and transforming with

respect to pulse length [4]. TMR uses 2nd- and 4th-order gradients to define a

homogeneous region in space from which the surface coil can detect a signal. The fatter two methods make

use of the inherent nonuniformity of B, along the “depth axis” to permit either the generation of a

selective excitation pulse, or to provide a variable which can be incremented (pulse width) and then transformed to produce a data set resolved along that

axis.

imaging Derived Localisation Methods By its very definition, magnetic resonance imaging

(MRI) produces focafised information; it is therefore an obvious development of imaging techniques and add chemical information either by preconditioning the spin system prior to obtaining the image or adding a further domain (i.e. 3-D).

Sensitive Region Methods Imaging techniques can be used to define a sensitive

region either by oscillating field gradients (sensitive

point) [5), slice selection techniques (DRESS) [6] or muftipufses [7, 81. A preconditioning method is inver- sion [i.e applying a nonselective l8Oo pulse to the imaging plane (or volume) and applying the excitation (read) 90° pulse In 2T,” later); this will produce and image with essentially no contribution from the species x whose spin lattice relaxation time (T,” is used to calculate the delay.

The selective saturation method also selectively

retains one species from the image but achieves this by applying a frequency-selective pulse immediately prior to the excitation pulse [9]. 3-D Fourier methods (i.e. x, y, 6): Information can be coded onto the data in two ways using a standard Fourier imaging sequence; i.e. phase encoded inbetween the excitation and the start of the read period, or frequency encoded during read period. The two methods have different properties: (1) Phase encoding can be performed for many variables but increases the imaging time by TR for each value of

each variable. (2) Frequency encoding is applicable to only one variable but increasing the number of points has essentially no effect on imaging time; for the frequency ranges used in imaging the sampling period required (= number of data points/2 max frequency) is very much less than TR. The allocation of informa- tion between the two modes therefore depends on the number of data points required to adequately define that variable. In order to minimise time, the variable requiring maximum data points is frequency encoded. If high-quality spectra are required then the chemical

shift information is recovered from the frequency

information, i.e. phase encode for x and y and collect

the “read data” without a gradient. This is true

chemical shift imaging [lo]. Alternatively, if only two points are required on the axis (e.g. to resolve only water and fat) then this information is phase encoded; for example, by changing the echo time, the “Dixon” or chemical shift selective method [ 111.

The lecture will compare and contrast these various methods from the viewpoint of efficiency selectively etc.

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Real Time Scanning of Babies by NMR (EPI)

P. Small,* A. Chrispin.? P. Mansfiefd,$ M. Doy1e.f B.

Chapmanf D. Guiffoyfe,$ M. Cawley,$ N. Rutter,* R. Coupfand.*

*University Hospital, TCity Hospital, $ University of Nottingham, Nottingham, England

Echo planar imaging (EPI) is a unique variant of NMR which enables the acquisition of cross-sectional

images in 35-65 msec. Rapid processing of the data enables a series of images to be produced which can be observed in real time and storage of this data will allow a sequence of ECG grated transections to form a “movie loop.” The rapidity with which EPI can acquire images makes it possible to take up to 32

contiguous slices to enable construction of saggitaf or coronal images from the stored data which can also be animated. The slice thickness is 7 mm and resolution 3 mm. The rapid motion of the infant thorax provides difficulties for conventional imaging modalities and may require general anaesthesia or sedation. None of the infants we have studied required any form of sedation and most have had marked tachycardia and tachypnoea, several being cyanosed at rest. Total scan- ning time was between 4 and 10 min.